Hong Kong’s leader has said it was “reasonable” to stop protesters holding pro-independence banners entering the forecourt of government headquarters after a rally on Monday.

Chief Executive Carrie Lam Cheng Yuet-ngor was referring to a pro-democracy march, organised by the Civil Human Rights Front, from Causeway Bay to the forecourt at Tamar.

“It is impossible for the government to accept the promotion of Hong Kong independence on its premises … so the [government’s] administrative wing and security guards’ warnings were reasonable and natural,” she said.

Monday’s National Day rally drew a crowd of about 1,500 people, according to organisers, demanding an inquiry into recent scandals surrounding the city’s most expensive rail project, the HK$97.1 billion (US$12.4 billion) Sha Tin-Central link, as well as protesting against the government’s unprecedented ban of the pro-independence Hong Kong National Party (HKNP).

Police estimated 1,256 people attended the march at its peak.

Outside government headquarters, minor scuffles broke out as a protester was stopped from entering the forecourt because the banner he was holding contained words that implied support for Hong Kong independence.

The slogan read: “If Hong Kong doesn’t go independent, it will become the mainland.”

The confrontation quickly escalated and some protesters pushed their way into the forecourt while police and security guards pushed back.

The protester, Leung Chi-hung, who was eventually allowed in, said he had taken the banner to previous protests and never been stopped.

“My slogan does not say anything about supporting Hong Kong independence. It only states the possible outcome if Hong Kong doesn’t go independent,” he said.

In a separate confrontation, security guards at government headquarters asked a protester to wrap up a banner about Hong Kong independence and scuffles broke out. Three security guards were reportedly pushed to the ground. They were injured and taken to hospital.

“It was extremely regrettable that some protesters ignored the guards’ warnings … and injured three [of my] colleagues,” Lam said on Tuesday.

Lam, who reopened the forecourt for public use at the beginning of her term last year, stressed the arrangement had come with certain rules.

The forecourt, nicknamed “Civic Square” by pro-democracy activists, was closed for public use by the government in July 2014 – as pro-democracy activists were discussing the occupation of roads or government premises as the last push for greater democracy in the city’s chief executive election.

After the HKNP was banned last week, some mainland Chinese officials and pro-Beijing politicians in Hong Kong renewed their call for the local government to enact national security legislation.

Lam reiterated the government would only do so when the “timing and atmosphere are appropriate”.

Under Article 23 of the Basic Law, the city’s mini-constitution, Hong Kong must enact its own law to prohibit acts such as “treason, secession, sedition” and subversion.

The last attempt to introduce the legislation in 2003 was shelved after half a million people took to the streets in protest, voicing worries about losing their civil liberties.